The specific, monoclonal antibodies produced by hybridoma technology may have important application to the study of erythroid differentiation. Using the techniques developed by Kohler and Milstein, we have applied this methodology in three broad areas to produce antibodies to interesting antigens: 1) hemoglobins A and F, antigens available in large quantity and high purity and for which antibodies have obvious applications in the measurement of specific hemoglobin types in human disease and in vitro culture; 2) erythropoietin and other growth factors, antigens which are available in very limited quantity and impure form for which specific antibodies may provide an important means of purification; and 3) antibodies to hematopoietic stem cells, especially the erythroid burst forming unit, the earliest human cell which is readily cultured in vitro. The inability to purify bone marrow stem cells to any degree of homogenity is the major obstacle in the study of hematopoietic differentiation.